Thermal management of electronic components and electronic boards including these components is essential to the successful operation of various aerospace vehicles, such as unmanned spacecraft. The continued miniaturization of the electronic components and integration schemes resulted in a dramatic increase of heat generated per unit volume. This increased heat generation not only limits the design of the circuit (e.g., the layout of electronic components on a board) and limits the design of individual electronic components but also jeopardizes the reliability of the overall circuit and individual components due to overheating of the components, connections, conductive lines, and other features of the electronic boards.
The heat generated during operation of these electronic components needs to be transferred to other areas to ensure continuous operations of the components. Because the boards often operate in vacuum environments, such as in unmanned spacecrafts, the heat may be primarily transferred through direct physical contact between various components, which may be referred to as conductive heat transfer. Specifically, the heat is transferred from components to a board supporting these components, then from the board to a chassis, and then from the chassis to a frame and other major components of the unmanned spacecraft.
Thermally conductive adhesives are often used to enhance the heat transfer between electronic components and boards supporting these components. A high thermal conductivity of these adhesives is essential for fast heat dissipation from the component and thermal management of the overall board. Currently available adhesives that meet specific requirements for space applications have a thermal conductivity of only up to 0.6 W/mK. These adhesives use Al2O3 (alumina) as a conductive filler suspended in an epoxy or polyurethane resin. Best thermally conductive but electrically insulative adhesives, which are not space compatible, have a thermal conductivity of only about 1 W/mK. These adhesives employ a combination of alumina, boron nitride, and/or aluminum nitride as a conductive filler suspended in an epoxy, polyurethane or silicone resin.
It should be noted that these values of thermal conductivity are based on measurements of a bonded joint configuration in a vacuum, which may be viewed as a modified ASTM C 177 test method performed in vacuum. Specifically, an adhesive material is used to bond two aluminum plates, and the heat flux between these plates is measured to determine the thermal conductivity value of the adhesive material. This method is believed to be the most representative of heat transfer between electronic components and boards supporting these components.
Other approaches used to measure a thermal conductivity include laser scanning of a free standing sample (e.g., a puck looking cylinder). The values obtained from these other measurement techniques are often different or, more specifically, larger than values obtained using the bonded joint configuration in vacuum, often 2-50 times larger. Furthermore, these other measurement techniques are not performed in vacuum resulting in artificially higher thermal conductivity values caused by additional heat losses through the air.